After considering several options the combination of complexity and vulnerability to water/soap scum lead me to capacitive sensing the Cat. For this I eventually settled on using Microchip's MTCH101 capacitive proximity part for detection. This provides a standalone detection to an alleged range of 6" and is dirt cheap ($0.44 at onsies). This part has an active low output and requires only a resistive divider to adjust the sensitivity.
12V solenoid valve (McMaster 7877K55) intended to hook up to the toilet's fresh water feed. The system consists of two boards which were etched using the sharpie resist method.